prometheus_text_format: Inline render_series/4 and render_value/2

These two functions are called once per metric series during every
scrape (~2.3M calls each in profiled RabbitMQ workloads), together
accounting for ~15.6% of scrape time. Inlining eliminates the
function call overhead and allows the compiler to optimize the
binary append operations across call boundaries.

Author: NelsonVides

src/formats/prometheus_text_format.erl

@@ -35,7 +35,7 @@ http_request_duration_milliseconds_sum{method=\"post\"} 4350
 -include("prometheus_model.hrl").
 
 -behaviour(prometheus_format).
--compile({inline, [render_label_pair/1]}).
+-compile({inline, [render_label_pair/1, render_series/4, render_value/2]}).
 
 ?DOC("""
 Returns content type of the latest \[text format](https://bit.ly/2cxSuJP).